Modulating and amplifying system



Aug. 19, 1941. w. HJBLISS 2,252,752

MODULATING AND AMPLIFYING SYSTEM Filed Aug. 31, 1938 ATTORNEY Patented Au 19, 1941 y r l MGDULATINGDAND AMPLIFYING SYSTEM Warren- BlissgBrooklyn, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Application August e1, 1938, Serial No. 227,644 comm e. (o 179--171.5)

This invention relates to systems tor use facsimile apparatus in the conversion of light of an image or subject of which the electro -optical representation is to be produced at receiving points into electric currents. The invention furtherrelates to the means by which these produced electric currents are caused; to energize a transmitter which is to distribute the produced energy either by radio or a wire line connectingrchannel to the appropriately selected receiving points.

In the conversion of: record subjects with. the usual form of facsimileapparatus the record. is carried upon a scanning and is illumihated in a point to point manner trom a suitable light source from which the light raysare directed I by a suitable optical through appropriately selected diaphragms to focus asa sharp point on the record subject. The light "reflected from the record subject is then: caused to'eiiergive the photosensitive element-of a photoelectrically active light translating element,

During the process of scanningrelative motion is provided between the illuminating light source and the subject. This relative motion may come about, for example, by causing the drum carrying the record subject to rotate while the tight spot is moved slowly longitudinally along the drum so that the record subject isstraversed in a. two-dimensional path'bythe light spot. Many other systems for causing a lei-dimensional analysis of the light values ofthe recordsubject are known andwill not hereiri be 'consideredgsihce the foregoing example may be taken'as purely illustrative of this phase of; the invention.

In the usual form of facsimile or picture transmitting system the amount ofli ght available is of relatively low'intensityvalu'e and, accordingly,

the photoelectric currents resulting from translating the varying light valuesof the record subject into electric currents are also relatively low; Such low intensity electric currents require a substantial degree of amplification prior to the time when the transmitter is modulated if the depth of modulation is to be adequate or incase the signals are used by way of a wire line="'oon-- nectlng channel to control directly a suitable recording device to reconstruct 'anelectro-optical likeness of the record subject which initially causes the production: of the electrical impulses,

the electric impulses must be of -relative great magnitude in order to produce satisfactory response of the recording means. t

Accordingly; the present invention has as one of its primary objects that of byprovicling greater ease in obtaining: amplification or the signals resu ting from the relatively low light values while at the same time providing an amplification system wherein wave form distortion of the electrical energy will not result because of the high degrees of amplification required.

Other objects of the invention are to provide a facsimile transmitting system and the operational circuits for use in connection-ltherewith which to a large extent make use of what is considered a novel manner of operating and connecting known elements so as to provide improved operation and higher degrees of efficiency in light translating operations 7 1 Other objects and advantages will, of course, naturally become apparent and at once suggest themselvesto those skilled in the art to which the invention is directed Wheat-he following specification and claims are read in connection the accompanying drawing, wherein the single figure thereof illustrates, largely indiagrammatic manner, one of the several sui table --formswhich the invention mayassuine in practieet l l Referring now to the drawing for a funtheruhderstanding of the invention it will be seen that itcomprises in its essential parts the use of suitable apparatus for directing light refiectedirom a subject orqrecord carried upon a-facsimile scami-in g drum upon an electronic lighttra nslatingelementt (such as a form of photoelectric tube) fro-m'which electrons of a magnitude proportional to the intensity of illiuriihating-light are released. Such a form of photoelectric tube m-a-y be one of the type already described'by Zworykin and others and known asan 'electronic multiplier tube (see Proceedings of the Institute of Radio- Engineers, March 1936, vol. 2d; Nos 3 at page 3511 et seq-l). The released electrons de- .veloped; within a photoelcctrically responsive. tube "of'this typeare, after a-considerable number of stages of multiplication, caused control-t a modulating circuit whichwprovisicn for compensating ior'hoh-li-nearities of the system is made.

Nowmaking more specific reference to thega'ccompanying drawing, a light beam it as reflected irom' the scanning drum aocordan'ce with the above outlined description, is caused total upon or to be projected upon thel-ightsensitive cathode or electrode element it of an'electrohic multiplier tube 4 5. One specifictype of. elec tronic multiplier tube whichis suitablefor this purpose may, for example be a tube oi the general type described. and claimedin Zworykin-and Massa Patent 2918304 granted April 27, 1937. .It comprises alplurality of electrode elements it,

' l1, l9 through 2|, arranged on one side of the tube and immediately above these electrode elements are a plurality of upper or accelerating electrodes 22. The accelerating electrodes are connected, as shown, to the next succeeding lower electrode and are maintained at the same potential. The final collecting electrode 23 is located immediately adjacent the last impact member 2| and intermediate the collector and the final lower electrode 2| is a screening electrode 24 which is connected with the last upper plate electrode 22, all as described in the aforesaid Zworykin et 2.1.

application. For the purpose of concentrating the electrons fiowing between the first lower electrode element l3 and the final collecting elec trode element 23 there is provided a suitable magnetic field developed by way of the electro magnet 25 which is energized from a source of direct current supplied from terminals 26. intensity of this electromagnetic field is controlled by means of the variable resistor 21. The magnetic field is arranged to become active transversely on the tube l5 as was described in the above named Zworykin et al. application.

Whenever light from the record drum passes along the path I I to activate or energize the lower electrode l3, photoelectrons are released therefrom and these electrons, in effect, bounce from one to another of the lower electrode members I3, l1, Hi to 2| etc. until they are collected by the final collector electrode 23. This then results in a greatly magnified electron flow in the final output circuit due to the release of secondary electrons from each of the electrode members l1, I9-2l etc. so that the current flowing in the final stage is increased over that resulting from the initial release of photoelectrons from the electrode I3 by the secondary emission factor of the tube raised to a power corresponding to the number of impact stages.

The output from the system hereinabove describedwhich is to be effective to control the modulator element may result from the voltage drop across a load resistor connected either to the final lower electrode element 2| or to the final collecting electrode 23, depending, of course, upon whether or not a control signal of positive or negative polarity, is desired. Therefore, con- 'nected to the final lower electrode element 2|, there is provided a resistor element 28 and similarly, connected to the final collecting electrode 23 there is provided a resistor 29. These resistors have their lower ends connected to points on voltage dividers 38 and 3! respectively, and then to ground at 32.

The system in the form disclosed by'the accompanying drawings is primarily designed for removing the signal from the multiplier tube across the resistor 28. For this purpose thevoltage drop appearing across the resistor 28 is transferred by way of conductor 36 to the control electrode circuit of the modulators 31 and 38 so as to act upon these tubes in parallel. Where it is desired to use a control signal of the opposite polarity the control voltage for the modulators 31 and 38 may be derived after suitably changing the bias on the tubes and other working parameters of the system by connecting the conductor 35 to the input or control electrode of the modulators 31 and 38 so that there is supplied to these tubes the voltage drop appearing across the output resistor 29.

The

pending upon which of the above described energizing paths for the modulators 31 and 38 is selected so as to energize the control grids of these tubes. The cathode elements of each of these modulator tubes 31, 38 are grounded at 33, while there is supplied to the screen grid electrodes 40 and 4| respectively a tone frequency by way of the transformer 42.

The tone frequency input which is to be modulated by the output energy from the tube I5 is applied to the terminal points 43. This tone frequency energy is developed from any suitable source of oscillatory energy (not shown) and is supplied for the purpose of permitting the use of a so-called A. C. amplifier in contrast to a straight D..C. amplifier. With the control energy from tube l5 applied to the control grid members of tubes 31 and 38 and the tone carrier supplied at the terminals 43 being fed from terminals 43 through the transformer 42 so as to become active upon the screen grid electrodes 40 and 4| of the tubes 31 and 38, the tone carrier frequency will be modulated in well known manner. It is usually desirable that the tone frequency be supplied to the tubes 31 and 38 so that these tubes operate as a balanced modulator. So connected, the secondary winding 44 of the transformer 42 is center-tapped at point 45 and this point connected by way of the conductor 48 to a suitable point on the'voltage divider 3|, as indicated. to supply the proper voltage upon the screen grid electrode members. Anode voltage for the tubes 31 and 38 is supplied by connecting the' conductor 41 to the righthand end of the voltage divider 3| and to the source of positive potential. This conductor 41 then connects'to the center-tap 48 of a potentiometer 49, having its outer ends connectedto the anode elements of tubes 31 and 38 through the conductors 58 and 40 5|. The potentiometer 49 serves'to balance the system in known manner.

- Output energy representing the amplified output from the tube I5 is then fed by way of the primary winding 52 of transformer 53 to the secondary winding 54 and thence caused to become effective upon the control electrode of a further amplifying tube 55. This amplifier tube '55 has its' anode voltage derived from the same source as the tubes 31 and 38 by way of a conductor 55 connected with the conductor 41 and to the primary winding 51 of the output transformer-58. There is also applied, however, to the amplifier tube 55 and caused also to become'effectivein the control electrode circuit thereof a counterelectromotive force derived from the carrier frequency input connected at the terminals 43 in order to improve the linearity of the system. This counter-electromotive force 7 is developed across the potentiometer 59 and is variable in its effect in accordance with the positioning of the slider contact 68 upon the potentiometer element 59. The slider contact 68 of'the potentiometer connects to one side of a condenser 5!, the other side of which is connected to the transformer secondary winding 54 so that there is applied in the control electrode circuit of thejtube 55 a counter-electromotive force derivedfrom the tone frequency input. This is the :voltage which is "used to improve the linear characteristics of the overall modulator device. The potential drop developed across resistor 62 is also eifective on the tube 55 by way of the connection of the control electrode circuit through resistor 82 and its connection to groundat 32. The

cathode element of the tube 55 is'connected to the ground point 32 by way of the usual shunt path comprising the cathode bias resistor 63 and condenser 64 and the amplified output energy from this tube is fed by Way of the primary Winding 51 of the transformer 58 to the secondary transformer winding 65. At the output terminals 66 the energy, as suitably amplified, may be used for the purpose of modulating the transmitter carrier or for keying with the carrier or, where desired, for controlling the operation of monitor equipment, or for the purpose, of actuating directly, or through suitable amplifiers, suitable reproducing equipment by which replicas of the subject causing the initial photo-response in the tube I may be made. Since the characteristics of tubes 31 and 38 may not be identical the positive half of the modulating cycle might vary from the negative half of the cycle, thus resulting in non-linearity of modulation. The potentiometer arrangement 59, 60 is such that a part of the carrier cycle is emphasized with respect to the other part of the cycle. This means, then, that a signal having opposite distortion characteristics to that developed in tubes 31 and 38 will be fed across the resistor 62 connected serially with the transformer secondary 54, thus compensating for nonlinearity in the modulating cycle.

While the present disclosure is not directed particularly to the exact form of transmitter which is to radiate the signal energy or to the wire-line connecting channels, which are to connect the amplifier 55 with suitable recording or monitoring apparatus, it is to be understood that this apparatus is of substantially standard form and does not per se form a part of the present invention. Any of the several well known forms of transmitter control may be utilized with the circuit and apparatus hereinabove described. Naturally, many and various modifications of the invention herein disclosed may be made Without departing from the spirit and scope thereof and, therefore, I believe myself to be entitled to make and use any and all such modifications which fall fairly within the spirit and scope of what is hereity of modulator tubes each having a plurality of cold electrode elements, means for applying in parallel modulation signals to one cold electrode of each of the modulator tubes, 2. source of carrier frequency energy, means for applying said carrier frequency energy to a second cold electrode of said modulator tubes in push-pull whereby said modulation energy is adapted to modulate the produced carrier frequency energy, an output circuit for said modulator tubes, an amplifier connected to receive upon its input circuit carrier frequency signal modulated energy from the modulator stage, means for simultaneously applying to the input circuit of the amplifier a counter-electromotive force derived from the carrier frequency source serially with the signal modulated energy, for improving linearity of the system, and a load circuit connected to the output of the amplifier.

2. A transmission system comprising a plurality of modulator tubes each having an electron emitter and a plurality of cold electrode elements, means for converting light energy into electrical energy and for applying the produced electrical energy as a modulation signal upon one cold electrode of each of the modulator tubes in parallel, a source of carrier frequency energy, means for applying said carrier frequency energy to a second cold electrode of each of said modulator tubes in phase opposition whereby said modulation energy is adapted to modulate the produced carrier frequency energy in each tube, an output circuit for receiving the modulated output from each of the modulator tubes, an amplifier connected to receive upon its input circuit carrier frequency signal modulated energy from each of the modulator tubes, means comprising a plurality of resistor and condenser elements for simultaneously applying to the input circuit of the amplifier a counter-electromotive force serially with the signal modulated energy derived from the carrier frequency source for improving linearity of the system, and a load circuit connected to the output of the amplifier.

3. A transmission system comprising a plurality of modulator tubes each having an electron emitter and a plurality of cold electrode elements, a photoelectrically responsive electron multiplier device for converting light energy into electrical energy and for applying the produced electrical energy as a modulation signal upon one cold electrode of each of the modulator tubes in parallel, a source of carrier frequency energy, means for applying said carrier frequency energy to a second cold electrode of each of said modulator tubes in phase opposition whereby said modulation energy is adapted to modulate the produced carrier frequency energy in each tube, an output circuit for receiving the modulated output from each of the modulator tubes, an amplifier connected to receive upon its input circuit carrier frequency signal modulated energy from each of the modulator tubes, means comprising a plurality of resistor and condenser elements for simultaneously applying to the input circuit of the amplifier a counter-electromotive force serially with the signal modulated energy derived from the carrier frequency source for improving linearity of the system, and a load circuit connected to the output of the amplifier.

4. A transmission system comprising means for developing electrical signals representative of intelligence, a balanced modulator, means for developing carrier frequency energy, means for simultaneously applying such carrier frequency energy and said signals representative of intelligence to said balanced modulator, an amplifying tube havin anode, cathode and at least one control electrode, means coupling the control electrode of said thermionic tube to the output of said balanced modulator whereby energy from the output circuit of said balanced modulator is impressed onto the control electrode-cathode circuit of said thermionic tube, resistance means connected serially with said coupling means, a potentiometer, means for impressing a wave of the same frequency as said carrier current onto said potentiometer, and means coupling said potentiometer to said serially connected resistance.

WARREN H. BLISS. 

